Generally, separation membranes are useful and economically efficient for separating materials, and many researches have been devoted to the development of them. In particular, a good many researches have been proceeded to find proper materials for membrane as well as proper processes for membrane separation. Until now, various kinds of membrane materials are developed and still are actively studied. Among imide type-polymer which is useful for support of composite membrane, polyetherimide is an excellent membrane material for water/alcohol separation or gas separation (q.v. "K. V. Peinemann: Journal of Membrane Science 27 (1986) 215-216").
Polyetherimide is developed by General Electric Company in a trademark of "ULTEM", which is noncrystalline thermoplastic and has very high glass transition temperature of 215.degree. C. This polyetherimide is considered as one of very important membrane materials having high mechanical strength and heat stability in addition to good separation degree. The preparation of asymmetric hollow fiber from polyetherimide in order to utilize such good properties in the field of separation membrane is reported (q.v. "K. Kneifel and K. V. Peinemann: Journal of Membrane Science, 65 (1992) 295307).
However, in case of preparing asymmetric hollow fiber using polyetherimide, there occurs a problem that pin holes form in the surface of the asymmetric hollow fiber membrane.
Asymmetric hollow fiber membrane consists of surface skin layer of dense structure and lower support layer having larger pores as shown in FIG. 1. Such an asymmetric hollow fiber membrane are subject to influence resulting from the change of conditions in the process of membrane preparation.
In the prior art, asymmetric hollow fiber membrane can be prepared by wet spinning or dry-wet spinning a solution of desired materials. The main factors having effect on the structure of asymmetric hollow fiber membrane are concentration and composition of polymer solution, solvent type of polymer, coagulating agent, spinning temperature or spinning height. Therefore, one can prepare asymmetric hollow fiber membrane having desired structure if he can control properly those factors in the course of preparing asymmetric hollow fiber membrane (qq.v "J. G. Wijmans, P. B. Baaij and C. A. Smolders: Journal of Membrane Science, 14 (1983) 266-274, D. M. Koenhen and H. M. Mulder and C. A. Smolders: Journal of Applied Polymer Science, 21 (1977) 199").
Asymmetric hollow fiber membrane thus prepared has several advantages: Firstly, asymmetric hollow fiber membrane has higher permeability over plain membrane of single material since it has thin skin layer, as shown in FIG. 1. At the same time, it can possess good mechanical strength without having harmful effect on permeability due to high porous, base support. Secondly, asymmetric hollow fiber can be used as a support for composite membrane since it is porous. Herein, a composite membrane means a membrane made by coating membrane material having good separation property thinly on high porous support. Those membranes have merits in improvement of permeability while possessing good separation property.
In spite of having those advantages, it is very difficult to prepare asymmetric hollow fiber membrane having thin skin layer having reduced numbers of pin holes on the surface of membrane. Because of that difficulty, most hollow fiber membrane presently available in the market are in the form of composite membrane using asymmetric hollow fiber. However, such a composite membrane has mechanical property inferior to single material membrane due to complicated process steps.
Therefore, there exists a need to provide a method for preparing asymmetric separation membrane having thin skin layer having reduced numbers of pin holes on the surface of membrane for separating water/organic solvent or gas.